还记得学校里学到的基本数学知识吗？就和它们一样。
| 例子 | 名称 | 结果 |
|---|---|---|
| -$a | 取反 | $a 的负值。 |
| $a + $b | 加法 | $a 和 $b 的和。 |
| $a - $b | 减法 | $a 和 $b 的差。 |
| $a * $b | 乘法 | $a 和 $b 的积。 |
| $a / $b | 除法 | $a 除以 $b 的商。 |
| $a % $b | 取模 | $a 除以 $b 的余数。 |
| $a ** $b | Exponentiation | Result of raising $a to the $b'th power. Introduced in PHP 5.6. |
除法运算符总是返回浮点数。只有在下列情况例外：两个操作数都是整数（或字符串转换成的整数）并且正好能整除，这时它返回一个整数。
取模运算符的操作数在运算之前都会转换成整数（除去小数部分）。
取模运算符 % 的结果和被除数的符号（正负号）相同。即 $a % $b 的结果和 $a 的符号相同。例如：
<?php
echo (5 % 3)."\n"; // prints 2
echo (5 % -3)."\n"; // prints 2
echo (-5 % 3)."\n"; // prints -2
echo (-5 % -3)."\n"; // prints -2
?>
参见手册中的数学函数。
To get a positiv result
function modulo(int $a, int $b):?int {
if ($b == 0) {
throw new Exception('modulo : second operand must not be zero');
}
$b = abs($b);
// test $b == 1 for performance when $a < 0
return ($b == 1) ? 0 : (($a < 0) ? modulo($a + $b, $b) : $a % $b);
}
If you need the mathematical modulo (always positive) from negative numbers, use this small function:
<?php
function modulo($a , $b) { return ($a + $b) % $b; }
// examples:
echo modulo(15, 12); // 3
echo modulo(-9, 12); // 3
?>
The modulus operator is very poorly suited for such a simple operation as determining if an int is even or odd. On most common systems, modulus performs a division, which is a very slow operation.
A much better way to find if a number is even or odd is to use the bitwise & operator.
e.g.
$is_odd = $x & 1; //using and
$is_odd = $x % 2; //using modulus
In addition to Jonathan's comment, there is a way simpler way to determine if an integer is even or not:
<? $odd = $i % 2; ?>
or
<? $even = !($i % 2); ?>
This works because a modulo division by 2 will always return either 0 or the rest 1. Since those are valid boolean values you can just invert them by adding a prefixed ! if wanted.
If you are running a php version older than 5.6, you can calculate $a ** $b by using exp($b*log($a))
The % operator doesn't behave as many people with a maths background would expect, when dealing with negative numbers. For example, -1 mod 8 = 7, but in PHP, -1 % 8 = -1.
The following function has the expected behaviour:
function mod($a, $n) {
return ($a % $n) + ($a < 0 ? $n : 0);
}
mod(-1, 8) returns 7 as expected.
For larger numbers (above PHP_INT_MAX), use fmod() rather than %.
The other operators (+-*/) work correctly with floats and integer overflow, but % uses integer wrap. Eg.
<?php
var_dump(0xffffffff % 2);
//Prints int(-1) which is WRONG
var_dump(intval(fmod(0xffffffff,2)));
//Prints int(1) which is the right answer
?>
(The reason this matters is that PHP's float is actually a double, and can accurately represent integers up to 52-bits, even on 32-bit systems)
It is worth noticing that when working with large numbers, most noticably using the modulo operator, the results depend on your CPU architecture. Therefore, running a decent 64-bit machine will be to your advantage in case you have to perform complex mathematical operations. Here is some example code - you can compare its output on x86 and x86_64 machines:
<?php
/* tested under PHP 5.2.6-1 with Suhosin-Patch 0.9.6.2 (cli) on both i386 and amd64, Debian lenny/sid */
$a = 2863311530;
$b = 256;
$c = $a % $b;
echo "$c <br />\n";
echo (2863311530 % 256)." <br />\n"; /* directly with no variables, just to be sure */
?>
The code is expected to produce '170' if working correctly (try it in spreadsheet software).
Be careful when using % with large numbers.
The code:
<?php
echo 3333333333 % 3
?>
puts out -1 instead of zero!
(Due to the overflow)
A very simple yet maybe not obvious use of the modulus (%) operator is to check if an integer is odd or even.
<?php
if (($a % 2) == 1)
{ echo "$a is odd." ;}
if (($a % 2) == 0)
{ echo "$a is even." ;}
?>
This is nice when you want to make alternating-color rows on a table, or divs.
<?php
for ($i = 1; $i <= 10; $i++) {
if(($i % 2) == 1) //odd
{echo "<div class=\"dark\">$i</div>";}
else //even
{echo "<div class=\"light\">$i</div>";}
}
?>
a real simple method to reset an integer to a the next lowest multiple of a divisor
$startSeq = $startSeq - ($startSeq % $entriesPerPage);
if $startSeq was already a multiple, then " $startSeq % $entriesPerPage " will return 0 and $startSeq will not change.
When dealing purely with HTML, especially tables, or other things in "grids" the modulous operator is really useful for splitting up the data with a seperator.
This snippet reads any gif files from the directory the script is in, prints them out and puts in a break every 5th image.
<?php
$d = dir('./');
$i = 0;
while(false !== ($e = $d->read())){
if(strpos($e,'.gif')){
++$i;
echo '<img src="'.$e.'"/>'.chr(10);
if(!($i%5))
echo '<br/>';
}
}
?>
For tables just put </tr><tr> in place of the break.
Note that operator % (modulus) works just with integers (between -214748348 and 2147483647) while fmod() works with short and large numbers.
Modulus with non integer numbers will give unpredictable results.